1. Field of the Invention
The present invention relates to an optical amplifier including a function of controlling a shutdown control speed.
2. Description of the Related Art
Recently, techniques including OADM (optical add drop multiplexing) have been applied to wavelength division multiplexing systems (WDM systems). OADM has particularly become an indispensable technique in a metro-access network connecting a user side and a core network. This is because OADM adds or drops an arbitrary wavelength to and from a wavelength-multiplexed signal, and thereby enables a communication capacity to be promptly changed according to the user, which allows a flexible network to be configured.
Also in this metro-access network, as with related arts, multi-stage optical amplifiers are employed in order to compensate for a loss in a transmission path. Because OADM is adopted, even if output from an upstream optical amplifier is stopped (hereinafter, referred to as “shutdown”) in a case where Add signal light is input into a downstream optical amplifier, output from the downstream optical amplifier is not shut down (in general, types of shutdowns include: a shutdown resulting from an interruption of input to an optical amplifier; and a shutdown resulting from factors other than the input interruption, that forcedly terminates output from an optical amplifier when a failure is caused on a transmission path side).
Accordingly, in a case where the speed of shutting down the upstream optical amplifier (more specifically, the speed of reducing the output level of the optical amplifier to zero. Hereinafter, referred to as “shutdown control speed”) is faster than the output control speed of the downstream optical amplifier, a transient level variation occurs in Add signal light output from the downstream optical amplifier. This will be described with reference to FIG. 1.
In FIG. 1, it is provided that on time t1 the upstream optical amplifier starts a shutdown in a state where signal light (λ1) from the upstream side and Add signal light (λ2) are input into the downstream optical amplifier controlling output so as to keep the gain constant.
Here, in a case where the shutdown control speed of the upstream optical amplifier is slower than (or equivalent to) the output control speed of the downstream optical amplifier, no transient level variation is caused in the Add signal light output from the downstream optical amplifier.
On the other hand, in a case where the shutdown control speed of the upstream optical amplifier is faster than the output control speed of the downstream optical amplifier, the downstream optical amplifier controls the gain constant with respect to the total of λ1 and λ2 and thus a transient level variation is caused in the Add signal light output from the downstream optical amplifier.
Therefore, in this viewpoint, it is preferable that the shutdown control speed of the upstream optical amplifier be slow.
On the other hand, in case of detection of an input interruption in the optical amplifier, it is expected that the optical amplifier will be immediately shut down in consideration of adverse influence on the human body. A lot of patent applications related thereto have been filed (e.g., JP2005-295157A etc.).
Accordingly, it has recently become a problem to satisfy the both of them, that is to say, to separately control the shutdown control speed based upon a case in which a shutdown results from an input interruption or based upon a case in which a shutdown results from factors other than an input interruption.